Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $40.76   3.371%   $42.134   $42.13   $42.15   $42.10  
  Purchase #2 $16.67   4.674%   $17.4492   $17.45   $17.45   $17.40  
  Purchase #3 $88.62   11.323%   $98.6544   $98.65   $98.65   $98.70  
  Purchase #4 $45.58   5.639%   $48.1503   $48.15   $48.15   $48.20  
  Purchase #5 $129.93   10.237%   $143.2309   $143.23   $143.25   $143.20  
  Purchase #6 $18.93   5.254%   $19.9246   $19.92   $19.90   $19.90  
  Purchase #7 $220.82   8.484%   $239.5544   $239.55   $239.55   $239.60  
  7 purchase totals = $561.31       $609.0978   $609.08   $609.10   $609.10